1. Field of the Invention
The present invention relates to an in-line four cylinder engine, and more particularly to an in-line four cylinder engine including a mechanism to reduce vibration caused by a secondary component of an inertia couple based on lateral pressures from the pistons.
2. Description of the Related Art
Engines include reciprocating engines, for example. A reciprocating engine includes a motion converter to convert reciprocating movements of the piston into rotational movements of the crankshaft (hereinafter referred to as a piston-crank mechanism).
Reciprocating engines include in-line four cylinder engines. An in-line four cylinder engine includes four cylinders arranged in the axial direction of the crankshaft, where the cylinder axes of the four cylinders are parallel or substantially parallel to each other.
In an in-line four cylinder engine, when the pistons in the two cylinders located at the ends of the series of cylinders arranged in the axial direction of the crankshaft are located at the top dead center, the pistons in the other two cylinders are located at the bottom dead center. This reduces vibration caused by the primary component of the inertia force generated as the piston-crank mechanism operates. More specifically, it reduces vibration caused by the primary component of the inertia force due to the reciprocating motion mass of the piston-crank mechanism acting as a translational force as well as vibration caused by the primary component of the same inertia force acting as a couple.
However, an in-line four cylinder engine with the above-described arrangement cannot reduce vibration caused by a secondary component of the inertia force generated as the piston-crank mechanism operates. In view of this, an in-line four cylinder engine includes a balancer to reduce vibration caused by the secondary component of the inertia force generated as the piston-crank mechanism operates. Such a balancer is disclosed in JP Sho56(1981)-7536 A, for example.
JP Sho56(1981)-7536 A discloses an in-line four cylinder engine including a secondary balancer device. According to this publication, the secondary balancer device includes two balancer shafts. The two balancer shafts are positioned parallel or substantially parallel to the crankshaft. The two balancer shafts rotate at a rotational speed twice that of the crankshaft. The two balancer shafts rotate in opposite directions. The two balancer shafts are located at different positions measured in the direction in which the cylinder axes of the four cylinders extend. One of the two balancer shafts is located closer to the pistons than the other balancer shaft is. The other balancer shaft rotates in the direction opposite to that in which the crankshaft rotates.
The secondary balancer device of JP Sho56(1981)-7536 A reduces not only vibration caused by the secondary component of the inertia force generated as the piston-crank mechanism operates acting as a translational force, but also vibration caused by a secondary component of the inertia couple based on the lateral pressures from the pistons (i.e., a secondary inertia couple).
In recent years, engines including a crankshaft having a shaft center offset from the cylinder axis have been proposed (such an engine will be hereinafter referred to as an offset engine). In an offset engine, as viewed in the axial direction of the crankshaft, the shaft center of the crankshaft is not located on the cylinder axis, and the angle or rotation of the crankshaft when the piston moves from its top dead center to its bottom dead center is larger than the angle of rotation of the crankshaft when the piston moves from its bottom dead center to its top dead center. Thus, the expansion time in an offset engine is longer than that in an engine including a crankshaft with a shaft center located on the cylinder axis (hereinafter referred to as a non-offset engine). Further, a reduction in piston speed during expansion means an improvement of heat efficiency. In addition, the inclination of the connecting rod when combustion pressure is high becomes smaller, reducing the lateral pressure from the cylinder from the piston, thus reducing loss. As a result, an offset engine improves fuel efficiency over a non-offset engine.